The present invention relates to a system for controlling an air-fuel ratio for an internal combustion engine emission control system having a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a value approximating the stoichiometric air-fuel ratio so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an oxygen sensor is provided to sense an oxygen content of exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of an air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the oxygen sensor with a predetermined value, a proportional and integrating circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the proportional and integrating circuit, and an on-off type electromagnetic valve for correcting the air-fuel ratio of the mixture. The control system operates to determine whether the feedback signal from the oxygen sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing a compensatory signal for actuating the on-off electromagnetic valve to thereby control the air-fuel ratio of the mixture.
The response of such a feedback control system is inherently slow because a detecting time by the oxygen sensor is delayed. More particularly, the mixture corrected by the on-off type electromagnetic valve is induced in the cylinder of the engine passing through the induction passage and burned therein, and thereafter discharged to the exhaust passage. Therefore, the time when the oxygen sensor detects the oxygen content of the exhaust gases based on the corrected mixture, a compensatory action with the on-off electromagnetic valve has overshot the desired point. As a result, a rich or lean mixture caused by the overshooting is induced in the engine and the deviation is detected by the oxygen sensor. Thus, the compensatory action in the opposite direction will be initiated. After such oscillation of the control operation, the variation of the air-fuel ratio of the mixture will converge toward the stoichiometric ratio. Therefore, the deviation of the air-fuel ratio of the mixture is corrected to the stoichiometric ratio with some delay.
If the control gain of the proportional and integrating circuit is decreased, the overshooting may be reduced in the steady state of the engine operation. However, the control is not sufficiently effected in the transient state such as acceleration or deceleration.
FIGS. 3A to 3C show output waveforms of the proportional and integrating circuit each of which is shifted by "S" in the transient state, and FIGS. 4A to 4C show output waveforms of the oxygen sensor corresponding to FIGS. 3A to 3C. FIG. 3A shows the waveform of the conventional control system and FIG. 3B shows the waveform of a system in which the control gain of the proportional and integrating circuit is reduced. The response time T.sub.B of FIG. 3B is longer than the response time T.sub.A of FIG. 3A. In the system of FIG. 3C, only the control gain of the integrating circuit is increased. The response time T.sub.C is smaller than the response time T.sub.A.
If the control gain of the circuit is reduced, the response time increases as shown in FIG. 3B. Therefore, the control will be delayed. On the other hand, if the control gain of the circuit is increased, the response time may be reduced as shown in FIG. 3C. However, the control operation is liable to overshoot.
On the other hand, Japanese Patent Application No. 54-98853 (U.S. patent application Ser. No. 174,385) (not prior art) discloses a system intended for improvement of such control delay of the conventional system, in which the oscillation center of a dither wave signal detected by the oxygen sensor is shifted according to the deviation of the output signal of the oxygen sensor for correcting the air-fuel ratio. However, when the engine is rapidly accelerated or decelerated, the correcting operation is delayed even in such a system.